Emissions from fossil fuel combustion facilities, such as flue gases of coal-fired utilities and municipal solid waste incinerators, typically include mercury. The emissions include vaporized mercury as elemental mercury, Hg0 as well as oxidized forms of mercury (Hg+2), such as in mercuric chloride or mercuric nitrate.
Many countries regulate or are contemplating regulations of emissions of mercury within waste gases because of potential environmental hazards posed by the mercury emissions. Hence, facilities that generate gas emissions that can contain mercury typically would monitor total mercury concentration in the emissions to comply with the regulations. For example, mercury monitoring systems can convert oxidized mercury to elemental mercury and then use atomic fluorescence spectroscopy to detect the elemental mercury. In atomic fluorescence spectroscopy, a spectrometer detects a concentration of a particular chemical species (e.g., an element or molecule) in a sample by measuring the degree to which that particular species absorb light of a wave-length which characterizes the species.
For example, to detect mercury within a gas emission sample, a light source emitting light at 253.7 nm is used to excite mercury atoms within a sample. As the elemental mercury within the gas sample absorbs the light from the light source, the elemental mercury enters an excited state. As the excited elemental mercury decays from the excited state back to a non-excited state, the elemental mercury releases energy by fluorescing light. A detector measures the light fluorescence produced by the sample. The fluorescence represents a measure of the concentration of the elemental mercury in the gas sample.
Certain conventional elemental mercury detectors utilize cold-vapor atomic absorption spectrometry (CVAAS) or cold-vapor atomic fluorescence spectrometry (CVAFS) as detection techniques. The CVAAS and CVAFS detection techniques, however, are susceptible to measurement interferences such as caused by interference gases (e.g., NOx, SO2, HCl, and Cl2) or quenching gases e.g., N2, O2, present within a sample. Elemental mercury detectors utilizing CVAAS or CVAFS detection techniques benefit from the removal of these interference gasses.
In the CVAAS technique, gases (e.g., NOx, SO2, HCl, and Cl2) may cause interference with the measurements made by associated elemental mercury detectors. The gasses absorb light during use of the CVAAS measurement technique. Thus, conventional elemental mercury detectors using the CVAAS measurement technique can provide a false reading. To minimize or remove interference gasses for detectors using the CVAAS technique, for example, elemental mercury detectors utilize a gold trap to minimize or remove the effects of SO2 within a gas sample. The gas sample flows, over time, through the gold trap, the gold material traps elemental mercury present within the gas sample. After the gold trap collects elemental mercury over time, the gold trap is heated and a SO2-free carrier gas is passed over the gold trap to deliver the elemental mercury collected on the gold trap to the detector. The gold trap, therefore, limits the effect of SO2 on the absorption of the elemental mercury and improves measurement sensitivity of the CVAAS detector.
For elemental mercury detectors using the CVAFS technique, fluorescence quenching by gases (e.g., N2, O2) can affect the performance of the detectors. In the CVAFS technique, concentrating devices, such as gold traps, are used to minimize or remove the effect of fluorescence quenching on the measurements made by the detectors. The trap collects elemental mercury over time and maximizes the detection sensitivity of the associated detector. The trapped mercury is then thermally desorbed into a gas stream of Argon, which is a much less efficient quencher than either nitrogen or oxygen. Thus the gas sample can be conditioned to minimize the presence ands effect of fluorescence quenching gases (e.g., N2, O2) on the measurements made by the detector using the CVAFS technique.